The long-term objective of this research is to examine the neural mechanisms underlying spatial attention and, in particular, different forms of spatial attention. Attention modulates the brain's representations of sensory stimuli and helps extract those aspects of the stimuli that are immediately relevant. For example, when searching for someone in a crowd, attention can be used to key in on a selected visual feature, such as color. Similarly, when listening to a symphony orchestra, attention allows us to focus on the music being played by particular sections (e.g., the violin section) of the orchestra. Besides working on individual senses, attention can cross stimulus-modality boundaries and link evens that involve different senses. For example, in the ventriloquism effect, if a person's lips are moving, their lips are perceived to be the source of the speech, regardless of the fact that the speech source originates elsewhere. While attention can be conceptualized as being uni-modal or cross-modal, it is unclear whether these forms of attention are mediated by common or distinct neural mechanisms. One hypothesis is that attention is controlled by a single "supra-modal" attentional system. An alternative hypothesis is that spatial attention results from the cooperation of different auditory and visual attentional system. Human studies, using psychophysical and electrophysiological techniques that were designed to test these alternative hypotheses have provided evidence in support of both hypotheses. This important issue can be probed further by comparing the responses of single neurons that are recorded while a monkey is engaged in tasks that probe uni-modal or cross-modal spatial attention. The advantage of this technique is that couples the high spatial and temporal resolution of single-unit recordings with the power of the awake, behaving monkey preparation. This proposal uses this technique to examine whether, in the lateral intraparietal area, attention is mediated through a supra-modal mechanism or through modality-dependent mechanisms. Since the macaque is a good model of human cortical function, this study has ramifications for the treatment of human disease. For example, underlying attention may increase our understanding of neuropathologies that have an attentional element such as neglect and schizophrenia.